The present invention relates to the field of compatible AM stereo receivers and more particularly to a means for improving performance by removing the correction factor during periods of low signal level.
In a stereo receiver for receiving a compatible AM stereo broadcast signal, a stereo corrector is used to restore the original left and right signals. Such a compatible signal is described in a co-pending application, Ser. No. 674,703, assigned to the same assignee as is the present invention.
In the transmitter of the above-referenced application, a carrier signal is modulated in quadrature by sum and difference signals. The combined quadrature signal is then amplitude limited to provide a signal having only phase information thereon. In a high level modulator stage, the limited carrier is again amplitude modulated, but with the sum signal only. Thus there is provided an output signal having only monophonic information on the envelope but having the correct phase information for stereo reception. In any stereo receiver for use with this signal, a correction factor must be utilized to restore exactly the original sum and difference signals. Since the broadcast signal differs from the original stereo signal (before limiting in the transmitter) by a factor equal to the cosine of the angle between a vector representing the sum signal and a vector representing the instantaneous quadrature signal, proper correction in a stereo receiver must include a division by this cosine correction factor.
With any reasonable signal, the cosine division will provide accurate stereo with no side effects. However, if a significantly weak or noisy signal is received, there will not only be noise in the demodulated (L-R)cos .phi. channel, but also a noise effect in the correction factor. Thus the process of correction could multiply the effect of the noise, further degrading the received signal. It is therefore desirable, though not necessary, to prevent division by the correction factor in a stereo receiver during periods of weak received signal.